Arterial Blood Gas

Question 1

What is the Alveolar Gas Equation

Answer 1

Patmos = 101/3 kPa
PH2O = 6.3 kPa
RQ = 0.8

Question 2

When commonly can the alveolar gas equation be used clinically and how is it used

Answer 2

To exclude the possibility of hypoventilation-associated hypoxia (i.e. where the concentration of CO2 is so high that it abuses Dalton’s Law to crowd O2 out of the alveolar gas mixture)

It also explains hypoxia on mount everest

It allows us to calculate our A-a gradient

Question 3

Define the following:

Daltons law
Partial pressure

Answer 3

Dalton’s Law:

The total pressure of a mixture of gases is equal to the sum of the partial pressures of all of the constituent gases

Definition of partial pressure

In a mixture of gases, partial pressure is the pressure that a gas would have exerted if it had occupied that volume alone.

Question 4

Henry’s law
Dissolved gas tension

Answer 4

Henry’s Law:

The amount of a given gas dissolved in a given liquid is directly proportional to the
partial pressure of the gas in contact with the liquid.
For each gas and each liquid, the proportionality constant (Henry’s constant) is different.
For any given partial pressure of a gas, the solubility will be inversely proportional to temperature.

Definition of dissolved gas tension

Gas tension of a solution is the partial pressure of a gas with which that solution is in equilibrium

Question 5

What is the p50 on the OHDC. Why is this concept important? what is normal p50

Answer 5

The p50 is the partial pressure of oxygen at which the Hb is 50% saturated with oxygen. The p50 value determines if the OHDC is left or right shifted.

Normal p50 about 3

Question 6

What causes left and right shift of the OHDC

Answer 6

RIGHT shift

Acidosis
Increased PaCO2 (the Bohr Effect)
Increased temperature
Increased 2,3-DPG (eg. in pregnancy)
Sulfhaemoglobin

LEFT shift

Alkalosis
decreased PaCO2
Decreased temperature
Decreased 2,3-DPG (eg. in stored blood)
Carboxyhaemoglobin
Methaemoglobin

Question 7

Differentiate the Haldane and Bohr effect

Answer 7

Both have to do with OFFLOADING

The Bohr effect helps the metabolizing tissues release oxygen from oxyhemoglobin

Haldane effect helps the lungs release carbon dioxide from carboxyhemoglobin.

Question 8

Why do we use an A-a gradient

Answer 8

It differentiates alveolar hypoventilation from all other causes of hypoxia.

If A-a gradient is normal - hypoxia is from hypoventilation or altitude (low FiO2)

If A-a gradient is abnormal - hypoxia is from VQ mismatch or Shunt